The best available evidence suggests that its about 4,350 years old. For thousands of years before that, most anthropologists believe, families consisted of loosely organized groups of as many as 30 people, with several male leaders, multiple women shared by them, and children. As hunter-gatherers settled down into agrarian civilizations, society had a need for more stable arrangements. The first recorded evidence of marriage ceremonies uniting one woman and one man dates from about 2350 B.C., in Mesopotamia. Over the next several hundred years, marriage evolved into a widespread institution embraced by the ancient Hebrews, Greeks, and Romans. But back then, marriage had little to do with love or with religion.

What was it about, then?

Marriages primary purpose was to bind women to men, and thus guarantee that a mans children were truly his biological heirs. Through marriage, a woman became a mans property. In the betrothal ceremony of ancient Greece, a father would hand over his daughter with these words: I pledge my daughter for the purpose of producing legitimate offspring. Among the ancient Hebrews, men were free to take several wives; married Greeks and Romans were free to satisfy their sexual urges with concubines, prostitutes, and even teenage male lovers, while their wives were required to stay home and tend to the household. If wives failed to produce offspring, their husbands could give them back and marry someone else.

When did religion become involved?

As the Roman Catholic Church became a powerful institution in Europe, the blessings of a priest became a necessary step for a marriage to be legally recognized. By the eighth century, marriage was widely accepted in the Catholic church as a sacrament, or a ceremony to bestow Gods grace. At the Council of Trent in 1563, the sacramental nature of marriage was written into canon law.

Did this change the nature of marriage?

Church blessings did improve the lot of wives. Men were taught to show greater respect for their wives, and forbidden from divorcing them. Christian doctrine declared that the twain shall be one flesh, giving husband and wife exclusive access to each others body. This put new pressure on men to remain sexually faithful. But the church still held that men were the head of families, with their wives deferring to their wishes.

When did love enter the picture?

Later than you might think. For much of human history, couples were brought together for practical reasons, not because they fell in love. In time, of course, many marriage partners came to feel deep mutual love and devotion. But the idea of romantic love, as a motivating force for marriage, only goes as far back as the Middle Ages. Naturally, many scholars believe the concept was invented by the French. Its model was the knight who felt intense love for someone elses wife, as in the case of Sir Lancelot and King Arthurs wife, Queen Guinevere. Twelfth-century advice literature told men to woo the object of their desire by praising her eyes, hair, and lips. In the 13th century, Richard de Fournival, physician to the king of France, wrote Advice on Love, in which he suggested that a woman cast her love flirtatious glancesanything but a frank and open entreaty.

Did love change marriage?

It sure did. Marilyn Yalom, a Stanford historian and author of A History of the Wife, credits the concept of romantic love with giving women greater leverage in what had been a largely pragmatic transaction. Wives no longer existed solely to serve men. The romantic prince, in fact, sought to serve the woman he loved. Still, the notion that the husband owned the wife continued to hold sway for centuries. When colonists first came to Americaat a time when polygamy was still accepted in most parts of the worldthe husbands dominance was officially recognized under a legal doctrine called coverture, under which the new brides identity was absorbed into his. The bride gave up her name to symbolize the surrendering of her identity, and the husband suddenly became more important, as the official public representative of two people, not one. The rules were so strict that any American woman who married a foreigner immediately lost her citizenship.

How did this tradition change?

Women won the right to vote. When that happened, in 1920, the institution of marriage began a dramatic transformation. Suddenly, each union consisted of two full citizens, although tradition dictated that the husband still ruled the home. By the late 1960s, state laws forbidding interracial marriage had been thrown out, and the last states had dropped laws against the use of birth control. By the 1970s, the law finally recognized the concept of marital rape, which up to that point was inconceivable, as the husband owned his wifes sexuality. The idea that marriage is a private relationship for the fulfillment of two individuals is really very new, said historian Stephanie Coontz, author of The Way We Never Were: American Families and the Nostalgia Trap. Within the past 40 years, marriage has changed more than in the last 5,000.

Men who married men

Gay marriage is rare in historybut not unknown. The Roman emperor Nero, who ruled from A.D. 54 to 68, twice married men in formal wedding ceremonies, and forced the Imperial Court to treat them as his wives. In second- and third-century Rome, homosexual weddings became common enough that it worried the social commentator Juvenal, says Marilyn Yalom in A History of the Wife. Looka man of family and fortunebeing wed to a man! Juvenal wrote. Such things, before were very much older, will be done in public. He mocked such unions, saying that male brides would never be able to hold their husbands by having a baby. The Romans outlawed formal homosexual unions in the year 342. But Yale history professor John Boswell says hes found scattered evidence of homosexual unions after that time, including some that were recognized by Catholic and Greek Orthodox churches. In one 13th-century Greek Orthodox ceremony, the Order for Solemnisation of Same Sex Union, the celebrant asked God to grant the participants grace to love one another and to abide unhated and not a cause of scandal all the days of their lives, with the help of the Holy Mother of God and all thy saints.